Traditionally, wireless communication devices (also referred to as UEs, User Equipments) are configured to use one or more radio access technology (RAT), wherein the RATs are significantly different in terms of, for example, which frequency band they reside in, which type of modulation is used, and/or which approach is applied for achieving multiple access (e.g. time division multiple access, TDMA, code division multiple access, CDMA, orthogonal frequency division multiple access, OFDMA, etc.). A few examples include:                GSM (Global System for Mobile communication) and EDGE (Enhanced Data rates for GSM Evolution) that utilize TDMA in separate 200 kHz frequency bands with modulation based on GMSK (Gaussian Minimum Shift Keying) and/or PSK (Phase Shift Keying),        UMTS (Universal Mobile Telecommunication System) and HSPA (High Speed Multiple Access) that utilize WCDMA (Wideband CDMA) in 5 MHz frequency bands where different users are separated by spreading codes widening the signal bandwidth, and        UMTS LTE (UMTS Long Term Evolution) that utilizes orthogonal frequency division multiplex (OFDM), where data is allocated on different subcarriers in the frequency domain and transformed into the time domain via, e.g. an inverse discrete Fourier transform (IDFT).        
Since New Radio (NR) is based on the same technique as UMTS LTE, transceivers for these two RATs will typically be able to share some of their fundamental building blocks for signal reception and/or signal transmission. Also higher layer functionality will typically be possible to share to some extent. Thus, many of the functions for NR and UMTS LTE may typically be supported by the same hardware (HW), with functional differences (if any) may be implemented in software (SW).
Generally, and particularly for UMTS LTE and NR, a wireless communication device may operate in one of a number of different modes, e.g. an idle mode and one or more connected modes.
In an idle (or generally; inactive) mode, the wireless communication device typically performs (intra-RAT and inter-RAT) mobility measurements and monitors paging. In an idle mode, the wireless communication device typically attempts to minimize its energy consumption by spending as much time as possible in a low power state and only wake up for short durations of time for measurements and paging monitoring.
In a connected (or generally; active) mode, the wireless communication device is typically constantly transmitting/receiving (e.g. data and/or time/frequency tracking signals). In a connected mode, the wireless communication device typically needs to be ready for high-rate data reception at a short notice.
Some categories of UEs are configured for simultaneous operation in accordance with several RATs (e.g. LTE and NR), for example enabled as an instance of multi-RAT multiple connectivity. In one example of dual-RAT dual connectivity, the UE may be configured to operate simultaneously in UMTS LTE (e.g. to ensure moderate-rate data coverage and system information provision) and NR (e.g. to provide additional high-rate data transmission).
In multi-RAT multiple connectivity, multi-band operations may need to be accommodated since NR may use a wide range of frequency bands and a UE may support multiple NR bands in addition to legacy UMTS LTE frequency bands. For example, a single radio frequency (RF) HW setup in a UE may support several frequency bands in a same frequency region, and the UE may have separate power amplifiers (PAs) or a common PA for transmission in the several frequency bands. However, to support several frequency bands in different frequency regions (e.g. a sub 6 GHz region vs. a millimeter wavelength—mmW—region), separate RF circuitry is typically required; including separate PAs.
Thus, the UE may be subject to HW limitations in the context of multi-RAT multiple connectivity, e.g. concerning supported frequency bands as exemplified above. Alternatively or additionally, the UE may be subject to other HW limitations in the context of multi-RAT multiple connectivity, e.g. concerning maximum possibly transmission power. For example, each PA is associated with maximum power limitations with corresponding distortion and power consumption implications. Furthermore, each UE design is typically associated with intermodulation issues and combinations of spurious signals from different bands that may be inadvertently mixed in the RF circuitry.
Each frequency band used in multi-RAT multiple connectivity is typically also subject to associated EMI (electromagnetic interference) regulations. Such regulations may, for example, stipulate criteria regarding one or more of a total instantaneous power limit, a total average power limit during a predetermined period, a power spectral density limit, and a specific absorption rate (SAR) limit.
Thus, there are several prerequisites to consider when applying multi-RAT multiple connectivity. Transmission power may typically be allocated to the involved RATs according to some default pre-determined distribution that ensures that all pre-requisites are properly handled. However, such an approach is typically sub-optimal in terms of performance metrics such as throughput and/or capacity.
Therefore, there is a need for alternative approaches to transmission power management. Preferably, such approaches lead to improvements in terms of performance metrics such as throughput and/or capacity.
It should be noted that the references herein to the combination of the two RATs UMTS LTE and NR are merely illustrative and that similar problems and/or solutions may be equally applicable for other combinations of RATs in a simultaneously connected mode of a wireless communication device.